First slide

Viscosity

Question

30 N horizontal force is required to pull a plate of area $1 m^{2}$ with constant velocity of 2 m/s over a thin film of oil of thickness 0.25mm, then viscosity of the oil in $N - S / m^{2}$ , is

Easy

A

$375 \times 10^{- 5}$
B

$275 \times 10^{- 4}$
C

$8.25 \times 10^{- 6}$
D

$12.2 \times 10^{- 6}$

Solution

For thin film, viscous drag, $F_{D} = η A \frac{V}{d}$
$∴ η = \frac{F_{D} . d}{A . V} = \frac{30 \times 0.25 \times 10^{- 3}}{1 \times 2} \frac{N - S}{m^{2}} = 375 \times 10^{- 5} \frac{N - S}{m^{2}}$

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